JP4105991B2 - High strength welded structural steel with excellent high heat input weld HAZ toughness and method for producing the same - Google Patents
High strength welded structural steel with excellent high heat input weld HAZ toughness and method for producing the same Download PDFInfo
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Description
【０００１】
【発明の属する技術分野】
本発明は、建築、橋梁、造船、海洋構造物、ラインパイプ、建設機械などの溶接構造物として広く利用可能な、母材靭性と溶接部ＨＡＺ（熱影響部）靭性の両方に優れた４９０MPa級の引張強度を有する溶接構造物用鋼およびその製造方法に関するものである。
【０００２】
【従来の技術】
建築、橋梁、造船、海洋構造物など溶接構造物の脆性破壊防止の観点から、母材の靭性だけでなく、溶接部からの脆性破壊の発生抑制すなわち、使用される鋼板のＨＡＺ靱性の向上に関する研究が数多く報告されてきた。一般に、母材靭性の確保のためには最終のフェライト粒径を小さくすることが肝要であり、必要靭性レベルにより普通圧延、制御圧延、さらには制御圧延＋加速冷却が利用されてきた。その基本はＡｌＮやＴｉＮなどの高温で安定な窒化物をピニング粒子として用いて、まず母材の加熱オーステナイト（γ）粒径を微細化した上で、さらに圧延によりオーステナイト中にフェライトの核生成サイトを多数導入し、最終フェライト粒径を微細にすることにある。
【０００３】
したがって、このような母材の製造方法では、当然ながら窒化物の種類により熱間圧延前の再加熱温度を変える必要が生じたり、加熱γ粒径の変動から最終のフェライト粒径にも変化が生じ、結果的に、母材靭性にバラツキが生じることがしばしば起こる。一方、溶接部ＨＡＺ靭性も加熱γ粒径が入熱量によって異なることから、要求靭性値が高いほどその値を小さくする必要があるにも関わらず、近年では加熱γ粒径が大きくなる条件、すなわち溶接施工能率の向上の観点から、大入熱溶接（およそ２０kJ/mm以下）や超大入熱溶接（２０〜１５０kJ/mm）が実施される場合が増加している。大入熱溶接と超大入熱溶接の鋼板への影響の差異は、高温での滞留時間の差異に起因しており、特に超大入熱溶接ではその時間が極めて長時間であるために、結晶粒径が著しく粗大化する領域が広く、靱性の低下が著しくなる点にある。
【０００４】
以上のような母材靭性のバラツキと溶接部ＨＡＺ靭性の入熱依存性の問題点を回避する抜本的な方法として、母材組織および溶接部ＨＡＺ組織の加熱γ粒径を同一のピニング粒子によって制御し、両者の高温での粒成長を顕著に抑制することが有効と考えられる。これが実現できた場合は、母材靭性の安定性はもとより入熱が大きくなった場合にも溶接部ＨＡＺ靱性を十分に向上させることができる。また、母材の加熱γ粒径が著しく微細になる場合には、従来の制御圧延や加速冷却を用いることなく普通圧延でも同程度のフェライト粒径と母材靭性を付与できる可能性が出てくることから、本技術の確立は工業的価値が高い。
【０００５】
加熱γ粒径のピニング効果が最も期待できる粒子として、高温でも溶解しにくい酸化物や硫化物が考えられる。例えば、酸化物の導入方法としては鋼の溶製工程においてＴｉなどの脱酸元素を単独に添加する方法があるが、多くの場合に溶鋼保持中に酸化物の凝集合体がおこり粗大な酸化物の生成をもたらすことによりかえって鋼の清浄度を損ない靱性を低下させてしまうことが知られている。そのため、複合脱酸法などさまざまな工夫がなされているが、従来知られている方法では、高温での母材の加熱γ粒径、さらには溶接入熱が大きく、しかも冷却速度が極めて小さい場合［例えば、８００℃から５００℃までの冷却速度が１℃／ｓ以下］の加熱γ粒径および変態後に生成される粒界フェライトの結晶粒粗大化を完全に阻止しうるほどの技術は未だに確立されていない。
【０００６】
【発明が解決しようとする課題】
本発明者らは、酸化物（あるいは硫化物）を最大限に微細分散させた上で、さらに超大入熱溶接時の粒界フェライトの粗大化抑制技術を鋭意検討し、超大入熱溶接においても溶接部ＨＡＺ組織を均質に微細化させ、溶接部ＨＡＺ靭性に優れた高強度溶接構造用鋼の製造技術の確立を課題とした。
【０００７】
【課題を解決するための手段】
上記課題を解決するための本発明の要旨は、以下の通りである。
（１） 質量％で
Ｃ ：０．０１〜０．２０％
Ｓｉ：０．０２〜０．５０％
Ｍｎ：０．８５〜２．０％
Ｐ ：≦０．０３％
Ｓ ：０．０００１〜０．０３０％
Ｖ ：０．０７〜０．５０％
Ａｌ：０．０００５〜０．０５０％
Ｔｉ：０．００３〜０．０５０％
を含み、残部が鉄および不可避的不純物からなり、さらに、ＭｇとＣａを同時に含有し、それぞれが、Ｍｇは０．０００９〜０．００２５％の範囲、Ｃａは０．０００１〜０．００８５％の範囲で、且つ、その合計が０．００１０〜０．０１０％の範囲であることを特徴とする超大入熱溶接部ＨＡＺ靭性に優れた高強度溶接構造用鋼。
（２） 質量％で、さらに、
Ｃｕ：０．０５〜１．５０％
Ｎｉ：０．０５〜０．５３％
Ｃｒ：０．０２〜１．５０％
Ｍｏ：０．０２〜１．５０％
Ｎｂ：０．０００１〜０．２０％
Ｚｒ：０．０００１〜０．０５０％
Ｂ ：０．０００３〜０．００５０％
のうち１種または２種以上を含有する（１）記載の超大入熱溶接部ＨＡＺ靭性に優れた高強度溶接構造用鋼。
（３） 溶接部ＨＡＺ組織の加熱γ粒径（旧オーステナイト粒径）が溶接入熱によらず２００μｍ以下であり、かつ粒界フェライト粒径が５０μｍ以下であることを特徴とする（１）および（２）記載の超大入熱溶接部ＨＡＺ靭性に優れた高強度溶接構造用鋼。
（４） （１）または（２）記載の鋼と同一成分を有する鋼塊をＡｃ３点以上、１３５０℃以下に加熱後、再結晶温度域で熱間圧延した後、自然冷却することを特徴とする超大入熱溶接部ＨＡＺ靭性に優れた高強度溶接構造用鋼の製造方法。
（５） （１）または（２）記載の鋼と同一成分を有する鋼塊をＡｃ３点以上、１３５０℃以下に加熱後、再結晶温度域で熱間圧延し、さらに未再結晶温度域において累積圧下率で４０〜９０％の熱間圧延をした後、自然冷却することを特徴とする超大入熱溶接部ＨＡＺ靭性に優れた高強度溶接構造用鋼の製造方法。
（６） （１）または（２）記載の鋼と同一成分を有する鋼塊をＡｃ３点以上、１３５０℃以下に加熱後、再結晶温度域で熱間圧延し、さらに未再結晶温度域において累積圧下率で４０〜９０％の熱間圧延をした後、１〜６０℃／ｓｅｃの冷却速度で０〜６００℃まで冷却することを特徴とする超大入熱溶接部ＨＡＺ靭性に優れた高強度溶接構造用鋼の製造方法。
（７） （１）または（２）記載の鋼と同一成分を有する鋼塊をＡｃ３点以上、１３５０℃以下に加熱後、再結晶温度域で熱間圧延し、さらに未再結晶温度域において累積圧下率で４０〜９０％の熱間圧延をした後、１〜６０℃／ｓｅｃの冷却速度で０〜６００℃まで冷却し、引き続いて３００℃〜Ａｃ１点に加熱して焼戻し熱処理することを特徴とする超大入熱溶接部ＨＡＺ靭性に優れた高強度溶接構造用鋼の製造方法。
【０００８】
【発明の実施の形態】
Ｍｇ、Ｃａは、従来から強脱酸剤、脱硫剤として鋼の清浄度を高めることで、溶接熱影響部の靱性を向上させることが知られている。また、これら元素を含有する酸化物の分散を制御して、母材靭性および溶接部ＨＡＺ靱性の両方を向上させる技術として用いた例が特開２００３−４９２３７号公報に記載されている。
本発明者らは、同じようにＭｇ、Ｃａの強脱酸剤あるいは強力な硫化物生成能に着目し、これら元素の添加順序および量を制御することで、超大入熱溶接部ＨＡＺ組織の加熱γ粒径の微細化に効果を有する酸化物あるいは硫化物の微細分散が期待できる余地があり、この技術とＶ含有鋼に見られるＶＮ（あるいはＶＮ／ＭｎＳ）の粒内変態能との組合せにより、超大入熱かつ冷却速度が著しく小さい場合（例えば薄手のスキンプレート等）の溶接部ＨＡＺ靭性も著しく向上するものと考えた。
【０００９】
以下、本発明に関して詳細に説明する。
本発明者らは、Ｔｉを添加し弱脱酸した溶鋼中にＭｇあるいはＣａを添加した場合の酸化物の状態を系統的に調べた。その結果、Ｓｉ、Ｍｎによる脱酸後に、Ｔｉ添加、Ｍｇ（Ｃａ）添加の順に添加した場合に、あるいはＴｉ添加とＭｇ（Ｃａ）添加を同時に行い、さらに平衡状態になった状態で再度Ｍｇ（Ｃａ）を添加するというサイクルを行なうことで、Ｍｇ（あるいはＣａ）の酸化物あるいは硫化物が極めて微細に、かつ高密度に生成されることを見出した。このＭｇ添加の効果はＣａをＭｇの代わりに用いても同様に得られ、いずれの元素を添加した場合も添加元素を含む酸化物もしくは硫化物が生成され、その粒子径は０．００５〜０．５μｍ、粒子数は鋼中に１mm2当たり１００００個以上であり、強力なピニング力を有していることが確認され、溶接部ＨＡＺ組織の加熱γ粒径が溶接入熱によらず２００μｍ以下となる。一般的に、２００μｍ以下のような微細な加熱γ粒径の場合には、超大入熱溶接時のような冷却速度においては、粒界フェライトの核生成頻度が増大し、しかもその成長速度が粗大な加熱γ粒径の場合に比較して大きくなるのが普通である。したがって、前述したような冷却速度、すなわち１℃／ｓ以下のような場合には粒界フェライトが数１０μｍから１００μｍ程に成長し、この粗大化が著しく靭性レベルを劣化させることが確認されるようになっている。
【００１０】
本発明者らは、このＨＡＺ細粒鋼の粒界フェライトの成長抑制方法について鋭意検討した結果、Ｖ添加が極めて有効であるとの知見を得るに至った。これは、ＶＮのような粒内変態核としての作用による粒界フェライトの体積率低減効果とγ／α界面でのＶ（Ｃ，Ｎ）析出および微量の固溶Ｖ元素の異相界面偏析による粒界フェライトの成長抑制効果の重畳によって、粒界フェライトの成長抑制が達成されるものと推定される。以上のような加熱γ粒径の細粒化技術とＶ添加の組合せによる超大入熱かつ冷却速度が極端に小さい場合の靭性改善策は従来全く報告されていないものであり、工業的価値は高い。
【００１１】
本発明は、ＭｇあるいはＣａの介在物の存在状態とＶ添加によって達成される母材靭性と溶接部ＨＡＺ靱性の両方に優れた鋼材に関するものであり、加熱γ粒径の変化を極力抑えた画期的な技術である。すなわち、本発明の特徴は、母材の加熱γ粒径（旧オーステナイト粒径）が再加熱温度によらず１００μｍ以下であり、さらに溶接部ＨＡＺ組織の加熱γ粒径（旧オーステナイト粒径）が前述したように溶接入熱によらず２００μｍ以下であり、しかも超大入熱溶接部の冷却速度が極めて小さくなるような板厚が６０mm以下のような柱部材［例えば、スキンプレート］溶接のような場合にも、粒界フェライトの粒径が５０μｍ以下となり、これらのミクロ組織を反映して、母材靭性と溶接部ＨＡＺ靭性の両方に優れた高強度溶接構造用鋼を提供できる点にある。
【００１２】
本発明におけるＭｇとＣａの添加方法であるが、ＭｇとＣａの効果を詳細に吟味した結果、これまでに知見できていなかった現象として、これら元素を同時に添加した場合には、Ｖ添加による粒界フェライトの成長抑制効果が単独添加の場合に比べて助長されるということが明らかになった。この場合、前述した加熱γ粒径の細粒化には大きく影響しない。ＭｇとＣａの同時添加の効果はこれら元素の合計量が大きくなるほど顕著になるが、一方で、粗大な酸化物・硫化物が生成されることから、高靭性を得るためには上限値を設定する必要があり、その値は１００ｐｐｍである。また、同時添加の効果が発揮されるための最小量は０．０００３％である。
【００１３】
以下、本発明の成分の限定理由について述べる。
Ｃ：Ｃは鋼における母材強度を向上させる基本的な元素として欠かせない元素であり、その有効な下限として０．０１％以上の添加が必要であるが、０．２０％を越える過剰の添加では、鋼材の溶接性や靱性の低下を招くので、その上限を０．２０％とした。
Ｓｉ：Ｓｉは製鋼上脱酸元素として必要な元素であり、鋼中に０．０２％以上の添加が必要であるが、０．５％を越えるとＨＡＺ靱性を低下させるのでそれを上限とする。
Ｍｎ：Ｍｎは、母材の強度および靱性の確保に必要な元素であるが、２．０％を越えるとＨＡＺ靱性を著しく阻害するが、逆に０．８％未満では、母材の強度確保が困難になるために、その範囲を０．８〜２．０％とする。
【００１４】
Ｐ：Ｐは鋼の靱性に影響を与える元素であり、０．０３％を越えて含有すると鋼材の母材だけでなくＨＡＺの靱性を著しく阻害するのでその含有される上限を０．０３％とした。
Ｓ：Ｓは０．０３０％を越えて過剰に添加されると粗大な硫化物の生成の原因となり、靱性を阻害するが、その含有量が０．００２５％未満になると、粒内フェライトの生成に有効なＭｎＳ等の硫化物生成量が著しく低下するために、０．００２５〜０．０３０％をその範囲とする。
【００１５】
Ｖ：Ｖは、本発明の主たる元素であり、窒化物［ＶＮ］形成元素として添加され、粒内フェライトの核として作用する。このとき、化学量論的にＮよりも過剰に添加することにより、炭化物あるいは炭窒化物の析出と固溶Ｖの確保により、粒界フェライトの成長が顕著に抑制される。これらの効果は、０．０７％以下の添加では十分でなく、０．５０％を越える添加では、逆に靱性の低下を招くために、その範囲を０．０７〜０．５０％以下とする。
【００１６】
Ａｌ：Ａｌは通常脱酸剤として添加されるが、本発明においては、０．０５％越えて添加されるとＭｇ、Ｃａの添加の効果を阻害するために、これを上限とする。また、Ｍｇ、Ｃａの酸化物を安定に生成するためには０．０００５％は必要であり、これを下限とした。
Ｔｉ：Ｔｉは、脱酸剤として、さらには窒化物形成元素としてし結晶粒の細粒化に効果を発揮する元素であるが、多量の添加は炭化物の形成による靱性の著しい低下をもたらすために、その上限を０．０５０％にする必要があるが、所定の効果を得るためには０．００３％以上の添加が必要であり、その範囲を０．００３〜０．０５０％とする。
【００１７】
Ｍｇ：Ｍｇは本発明の主たる合金元素であり、主に脱酸剤あるいは硫化物生成元素として添加され、Ｃａと同時添加により顕著に粒界フェライトの成長を抑制する。その量は粒界フェライトの成長抑制効果の下限としてＣａ量との総和で０．００１０％、また上限値は粗大な酸化物生成による靭性低下をもたらすことから同様にＣａ量との総和で０．０１％とする。なお、単独のＭｇ量としては、０．００２５％を越えて添加されると、粗大な酸化物あるいは硫化物が生成し易くなり、母材およびＨＡＺ靱性の低下をもたらす。一方で、０．０００９％未満の添加では、ピニング粒子として必要な酸化物の生成が十分に期待できなくなるため、その添加範囲を０．０００９〜０．００２５％と限定することが望ましい。
Ｃａ：Ｃａは硫化物を生成することにより伸長ＭｎＳの生成を抑制し、鋼材の板厚方向の特性、特に耐ラメラティアー性を改善する。さらに、Ｃａは前述のようにＭｇとの同時添加により、粒界フェライトの成長抑制効果を有していることから、本発明の重要な元素である。Ｃａの範囲はＭｇと同じ理由により、その範囲は０．０００１％〜０．００８５％の範囲に限定することが望ましく、さらにＭｇ量との関係を考慮して、その合計を０．００１０〜０．０１％に限定する必要がある。
【００１８】
なお、本発明においては、強度および靱性を改善する元素として、Ｃｕ、Ｎｉ、Ｃｒ、Ｍｏ、Ｎｂ、Ｚｒ、Ｂの中で、１種または２種以上の元素を必要に応じて添加することができる。
【００１９】
Ｃｕ：Ｃｕは、靱性を低下させずに強度の上昇に有効な元素であるが、０．０５％未満では効果がなく、１．５％を越えると鋼片加熱時や溶接時に割れを生じやすくする。従って、その含有量を０．０５〜１．５％以下とする。
Ｎｉ：Ｎｉは、靱性および強度の改善に有効な元素であり、その効果を得るためには０．０５％以上の添加が必要であるが、０．５３％以上の添加では溶接性が低下するために、その上限を０．５３％とする。
Ｃｒ：Ｃｒは析出強化による鋼の強度を向上させるために、０．０２％以上の添加が有効であるが、多量に添加すると、焼入れ性を上昇させ、ベイナイト組織を生じさせ、靱性を低下させる。従って、その上限を１．５％とする。
【００２０】
Ｍｏ：Ｍｏは、焼入れ性を向上させると同時に、炭窒化物を形成し強度を改善する元素であり、その効果を得るためには、０．０２％以上の添加が必要になるが、１．５０％を越えた多量の添加は必要以上の強化とともに、靱性の著しい低下をもたらすために、その範囲を０．０２〜０．５０％以下とする。
Ｎｂ：Ｎｂは、炭化物、窒化物を形成し強度の向上に効果がある元素であるが、０．０００１％以下の添加ではその効果がなく、０．２０％を越える添加では、靱性の低下を招くために、その範囲を０．０００１〜０．２０％以下とする。
Ｚｒ：ＺｒもＮｂと同様に炭化物、窒化物を形成し強度の向上に効果がある元素であるが、０．０００１％以下の添加ではその効果がなく、０．０５０％を越える添加では、逆に靱性の低下を招くために、その範囲を０．０００１〜０．０５０％以下とする。
【００２１】
Ｂ：Ｂは一般に、固溶すると焼入れ性を増加させるが、またＢＮとして固溶Ｎを低下させ、溶接熱影響部の靱性を向上させる元素である。従って、０．０００３％以上の添加でその効果を利用できるが、過剰の添加は、靱性の低下を招くために、その上限を０．００５０％とする。
【００２２】
上記の成分を含有する鋼は、製鋼工程で溶製後、連続鋳造などを経て再加熱、圧延、冷却処理を施される。この場合、以下の点を限定した。
熱間圧延・制御圧延ともに、鋼塊をオーステナイト化するためにＡｃ3点以上の温度に加熱する必要がある。しかし、１３５０℃を超えて加熱すると、熱源コストの増大が生じることから、加熱温度は１３５０℃以下とした。
次いで、熱間圧延・制御圧延ともに、再結晶温度域で圧延することによりオーステナイト粒径を小さくすることが必要である。また、制御圧延を用いて、強度上昇と靭性向上を図る場合には、さらに未再結晶温度域で圧延することによりオーステナイト粒内に変形帯を導入し、フェライト変態核を導入することが有効である。未再結晶域での累積圧下率が４０％未満では変形帯が十分に形成されないので、未再結晶域で累積圧下率の下限値を４０％とした。しかし、累積圧下率が９０％を超えると、母材シャルピー試験の吸収エネルギーの低下が著しくなるために、上限を９０％にした。
【００２３】
自然放冷よりさらに強度を上昇させるためには加速冷却が必要である。しかしながら、冷却速度が１℃/sec未満では、十分な強度を得ることができない。逆に、冷却速度が６０℃/sec超ではベイナイト主体組織が生成するため母材の靭性が低下する。したがって、冷却速度を１〜６０℃/secに限定した。本発明においては、母材の強度を得るために変態が終了するまで加速冷却を継続する必要がある。このため、冷却停止温度の上限を６００℃とした。６００℃超の停止温度では変態が終了しないために、十分な強度が得られない。通常、加速冷却は水を冷却媒体として用いる。それ故、実際上の冷却停止温度の下限は０℃となるので、下限値を０℃とした。
【００２４】
加速冷却後の焼戻し熱処理は回復による母材組織の靭性向上を目的としたものであるから、加熱温度は逆変態が生じない温度域であるＡｃ1点以下でなければならない。回復は転位の消滅・合体により格子欠陥密度を減少させるものであり、これを実現するためには３００℃以上に加熱することが必要である。このため、加熱温度の下限を３００℃とした。上限は変態点以下であるため、ＡＣ1を上限とした。
【００２５】
【実施例】
次に、本発明の実施例について述べる。
表１の化学成分を有する鋼塊を表２に示す製造条件により、板厚１２mm〜１００mmの厚鋼板とした後、溶接入熱が１００kJ/mm、８００℃から５００℃での冷却速度が０．３５℃／ｓの超大入熱かつ超緩冷却の溶接を付与し、旧γ粒と粒界フェライトのそれぞれの粒径を測定するとともに、溶接部ＨＡＺ靭性を０℃におけるシャルピー吸収エネルギーにより評価した。なお、母材靭性については、加熱温度を１１００℃〜１３５０℃の範囲で５水準の温度にて製造しているが、全て良好な母材靭性であった。発明鋼の延性・脆性遷移温度（vＴrs）は−４０℃以下であり、試験温度−４０〜−８０℃の範囲にて高いシャルピー吸収エネルギー値（１００Ｊ以上）を示した。次に、溶接部のＨＡＺ靭性について記述する。
【００２６】
まず、鋼１〜６は本発明の例を示したものである。表２に示すように、本発明の鋼板は化学成分と製造条件の各要件を満足しており、ＨＡＺの加熱γ粒径が２００μｍ以下となっていることに加えて、冷却中に旧γ粒界にそって生成した粒界フェライトの粒径がいずれも５０μｍ以下であり、高いＨＡＺ靭性を有していることがわかる。
【００２７】
それに対し、鋼７〜１９は本発明方法から逸脱した比較例である。すなわち、鋼７〜１１は基本成分あるいは選択元素の内いずれかの元素が、発明の要件を越えて添加されている例であり、本発明の重要な論点であるＶ量の確保と「ＭｇとＣａの合計量」に関す要件を満たしているものの靱性劣化要因となる元素が過剰に添加された事により超大入熱ＨＡＺ靱性の劣化が大きい。鋼１２〜１５ではＶとＡｌがいずれも下限値ないしは上限値を逸脱した場合に相当している。順に特性を見ると、まず鋼１２は粒界フェライトが大きいことによって靭性値が低くなっている。鋼１３はＶ量が著しく高いために、やはり靭性値が劣化していることがわかる。鋼１４と鋼１５はＡｌ量の影響が大きいことを示しており、これらの場合も靭性は低い。次に、鋼１６〜鋼１８はいずれもＭｇ＋Ｃａの合計量が範囲外になっている例である。鋼１６はこれら元素の不足のために、粒界フェライトが粗大化している場合であり、一方、鋼１７と１８は過剰添加によって粒界フェライトが微細になっているにも関わらず、５μｍ以上の粗大酸化物数が増大したことにより靭性値が大きく低下している。
鋼１９はＶ、Ｍｇ、Ｃａの３元素がいずれも添加されていない場合であり、他に比べて著しく粒界フェライトが粗大化しており、ＨＡＺ靭性が最も悪い。
【００２８】
【表１】
【表２】
【発明の効果】
本発明の化学成分および製造方法に限定し、ＭｇとＣａを同時添加することで、母材の加熱γ粒径を微細化することができ、さらにＶ添加との組合せによって溶接入熱に関わらずＨＡＺの加熱γ粒径だけでなく靭性に悪影響を及ぼす粒界フェライトを同時に微細化することができ、これらの微細化効果により母材靭性と溶接部ＨＡＺ靱性の両者に優れた画期的な高強度溶接構造用鋼の製造が可能となる。その結果、建築、橋梁、造船、海洋構造物、ラインパイプ、建設機械などの溶接構造物の脆性破壊に対する安全性が大幅に向上し、産業上の効果は著しく大きい。[0001]
BACKGROUND OF THE INVENTION
The present invention can be widely used as a welded structure such as a building, a bridge, a shipbuilding, an offshore structure, a line pipe, and a construction machine, and is excellent in both base metal toughness and welded portion HAZ (heat affected zone) toughness. The present invention relates to a steel for welded structures having a tensile strength of 5 and a method for producing the same.
[0002]
[Prior art]
From the viewpoint of preventing brittle fracture of welded structures such as architecture, bridges, shipbuilding, and offshore structures, not only the toughness of the base metal but also the suppression of the occurrence of brittle fracture from the welded part, that is, the improvement of the HAZ toughness of the steel sheet used Many studies have been reported. In general, it is important to reduce the final ferrite grain size in order to ensure the toughness of the base metal. Depending on the required toughness level, normal rolling, controlled rolling, and controlled rolling + accelerated cooling have been used. The basics are to use high-temperature stable nitrides such as AlN and TiN as pinning particles, first refine the heated austenite (γ) grain size of the base material, and then roll the ferrite nucleation sites in the austenite In order to make the final ferrite grain size fine, a large number of them are introduced.
[0003]
Therefore, in such a manufacturing method of the base material, it is natural that the reheating temperature before hot rolling needs to be changed depending on the type of nitride, or the final ferrite particle size changes due to the variation of the heated γ particle size. Often results in variations in the base material toughness. On the other hand, the weld HAZ toughness also varies depending on the amount of heat input, so that the higher the required toughness value, the smaller the value needs to be reduced in recent years. From the viewpoint of improving the welding work efficiency, cases where large heat input welding (approximately 20 kJ / mm or less) and super large heat input welding (20 to 150 kJ / mm) are increasing. The difference in the effect of high heat input welding and super high heat input welding on the steel sheet is due to the difference in residence time at high temperatures, and especially in super high heat input welding, the time is extremely long. The area where the diameter is remarkably coarsened is wide and the toughness is remarkably lowered.
[0004]
As a fundamental method for avoiding the above-mentioned variation in base metal toughness and heat input dependence of welded part HAZ toughness, the heating γ grain size of the base metal structure and welded part HAZ structure is determined by the same pinning particles. It is considered effective to control and remarkably suppress the grain growth at both high temperatures. If this can be realized, the weld HAZ toughness can be sufficiently improved not only in the stability of the base metal toughness but also in the case where the heat input becomes large. In addition, when the heated γ grain size of the base material becomes extremely fine, there is a possibility that the same ferrite grain size and base material toughness can be imparted even with ordinary rolling without using conventional controlled rolling or accelerated cooling. Therefore, the establishment of this technology has high industrial value.
[0005]
Oxides and sulfides that are difficult to dissolve even at high temperatures can be considered as the particles that are most expected to have the pinning effect of the heated γ particle diameter. For example, as a method for introducing an oxide, there is a method in which a deoxidizing element such as Ti is added alone in the steel melting process. In many cases, however, the oxide is a coarse oxide due to aggregation and coalescence of the oxide during holding of the molten steel. On the other hand, it is known that the cleanliness of the steel is impaired and the toughness is lowered by causing the formation of. For this reason, various measures such as a composite deoxidation method have been made. However, in the known method, the heating γ particle size of the base material at a high temperature, the welding heat input is large, and the cooling rate is extremely low. [For example, the cooling rate from 800 ° C. to 500 ° C. is 1 ° C./s or less] The technology that can completely prevent the grain size of the grain boundary ferrite formed after the heating γ grain size and transformation is still established. It has not been.
[0006]
[Problems to be solved by the invention]
The inventors of the present invention have made a fine dispersion of oxide (or sulfide) to the maximum extent, and further studied the technology for suppressing the coarsening of grain boundary ferrite during super-high heat input welding. The objective was to establish a manufacturing technology for high strength welded structural steel with excellent weld zone HAZ toughness by uniformly miniaturizing the weld zone HAZ structure.
[0007]
[Means for Solving the Problems]
The gist of the present invention for solving the above problems is as follows.
(1) By mass% C: 0.01 to 0.20%
Si: 0.02 to 0.50%
Mn: 0.85 to 2.0%
P: ≦ 0.03%
S: 0.0001 to 0.030%
V: 0.07 to 0.50%
Al: 0.0005 to 0.050%
Ti: 0.003 to 0.050%
And the balance consists of iron and inevitable impurities, and further contains Mg and Ca simultaneously, each of which Mg is in the range of 0.0009 to 0.0025% , Ca is 0.0001 to 0.0085% A high strength welded structural steel excellent in super high heat input weld HAZ toughness, characterized in that the total is in the range of 0.0010 to 0.010%.
(2) In mass%,
Cu: 0.05 to 1.50%
Ni: 0.05-0.53%
Cr: 0.02-1.50%
Mo: 0.02 to 1.50%
Nb: 0.0001 to 0.20%
Zr: 0.0001 to 0.050%
B: 0.0003 to 0.0050%
High strength welded structural steel excellent in super high heat input weld HAZ toughness according to (1), which contains one or more of them.
(3) The heated γ particle size (old austenite particle size) of the weld zone HAZ structure is 200 μm or less irrespective of welding heat input, and the grain boundary ferrite particle size is 50 μm or less (1) and (2) High strength welded structural steel excellent in HAZ toughness according to (2) super high heat input weld zone.
(4) A steel ingot having the same component as the steel described in (1) or (2) is heated to a temperature of Ac 3 or higher and 1350 ° C. or lower, then hot-rolled in a recrystallization temperature range, and then naturally cooled. A method for producing high strength welded structural steel with excellent high heat input weld HAZ toughness.
(5) After heating the steel ingot having the same component as the steel described in (1) or (2) to Ac 3 points or more and 1350 ° C. or less, it is hot-rolled in the recrystallization temperature range, and further in the non-recrystallization temperature range A method for producing high strength welded structural steel excellent in super high heat input weld HAZ toughness, characterized by performing natural cooling after hot rolling at a cumulative rolling reduction of 40 to 90%.
(6) A steel ingot having the same composition as the steel described in (1) or (2) is heated to Ac 3 points or more and 1350 ° C. or less, then hot-rolled in the recrystallization temperature range, and further in the non-recrystallization temperature range High strength with excellent super high heat input weld HAZ toughness, characterized by hot rolling at a cumulative rolling reduction of 40 to 90% and cooling to 0 to 600 ° C. at a cooling rate of 1 to 60 ° C./sec. Manufacturing method of steel for welded structure.
(7) After heating the steel ingot having the same component as the steel described in (1) or (2) to Ac 3 points or more and 1350 ° C. or less, it is hot-rolled in the recrystallization temperature range, and further in the non-recrystallization temperature range After hot rolling with a cumulative rolling reduction of 40 to 90%, it is cooled to 0 to 600 ° C. at a cooling rate of 1 to 60 ° C./sec, and subsequently heated to 300 ° C. to Ac 1 point for tempering heat treatment. A method for producing high strength welded structural steel excellent in HAZ toughness with super large heat input welds characterized by
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Conventionally, Mg and Ca are known to improve the toughness of the weld heat-affected zone by increasing the cleanliness of steel as a strong deoxidizer and desulfurizer. Japanese Patent Laid-Open No. 2003-49237 discloses an example in which the dispersion of oxides containing these elements is controlled to improve both the base metal toughness and the welded portion HAZ toughness.
In the same way, the present inventors pay attention to the strong deoxidizer of Mg and Ca or the strong sulfide-forming ability, and control the addition order and amount of these elements to heat the super large heat input weld zone HAZ structure. There is room to expect fine dispersion of oxides or sulfides that have an effect on the refinement of the γ grain size. By combining this technology with the intragranular transformation ability of VN (or VN / MnS) found in V-containing steels. It was considered that the weld HAZ toughness was also significantly improved when the super heat input and the cooling rate were extremely low (for example, a thin skin plate).
[0009]
Hereinafter, the present invention will be described in detail.
The present inventors systematically investigated the state of oxides when Mg or Ca was added to molten steel that was weakly deoxidized by adding Ti. As a result, after deoxidation with Si and Mn, when Ti addition and Mg (Ca) addition are added in this order, or Ti addition and Mg (Ca) addition are performed simultaneously, and Mg (Ca) is again in an equilibrium state. It has been found that by performing a cycle of adding Ca), an oxide or sulfide of Mg (or Ca) is generated extremely finely and with high density. The effect of adding Mg can be obtained in the same manner even when Ca is used instead of Mg. When any element is added, an oxide or sulfide containing the added element is generated, and the particle size is 0.005 to 0. .5 μm, the number of particles is 10,000 or more per mm 2 in steel, and it has been confirmed that it has a strong pinning force, and the heated γ particle size of the weld zone HAZ structure is 200 μm or less regardless of welding heat input. It becomes. In general, in the case of a fine heating γ grain size of 200 μm or less, the nucleation frequency of grain boundary ferrite increases at a cooling rate during super-high heat input welding, and the growth rate is coarse. Usually, it becomes larger than that of a heated γ particle size. Therefore, in the case of the cooling rate as described above, that is, 1 ° C./s or less, the grain boundary ferrite grows from several tens μm to 100 μm, and it is confirmed that this coarsening significantly deteriorates the toughness level. It has become.
[0010]
As a result of intensive studies on the method for suppressing the growth of intergranular ferrite in this HAZ fine-grained steel, the present inventors have obtained the knowledge that V addition is extremely effective. This is because of the effect of reducing the volume fraction of intergranular ferrite due to the action of intragranular transformation nuclei such as VN, V (C, N) precipitation at the γ / α interface, and segregation at the heterogeneous interface of a small amount of solid solution V element. It is presumed that the growth suppression of grain boundary ferrite is achieved by superimposing the growth suppression effect of boundary ferrite. There has been no report on toughness improvement in the case where the super heat input and the cooling rate are extremely small by the combination of the above-described refinement technique of heating γ grain size and V addition, and the industrial value is high. .
[0011]
The present invention relates to a steel material excellent in both the base metal toughness and the welded portion HAZ toughness achieved by the presence of inclusions of Mg or Ca and the addition of V. This is an innovative technology. That is, the feature of the present invention is that the heated γ particle size (old austenite particle size) of the base material is 100 μm or less regardless of the reheating temperature, and the heated γ particle size (old austenite particle size) of the weld HAZ structure is As described above, it is 200 μm or less regardless of welding heat input, and the column member whose thickness is 60 mm or less so that the cooling rate of the super-high heat input welding portion becomes extremely small (for example, skin plate) welding Even in this case, the grain size of the grain boundary ferrite is 50 μm or less, which reflects the microstructure and can provide a high-strength welded structural steel excellent in both base metal toughness and welded portion HAZ toughness.
[0012]
The method of adding Mg and Ca in the present invention. As a result of examining the effects of Mg and Ca in detail, as a phenomenon that has not been known so far, It has been clarified that the growth suppression effect of the boundary ferrite is promoted as compared with the case of adding alone. In this case, the heating γ particle size is not greatly affected. The effect of simultaneous addition of Mg and Ca becomes more prominent as the total amount of these elements increases, but on the other hand, since coarse oxides and sulfides are generated, an upper limit is set to obtain high toughness. The value is 100 ppm. Moreover, the minimum amount for exhibiting the effect of simultaneous addition is 0.0003%.
[0013]
Hereinafter, the reasons for limiting the components of the present invention will be described.
C: C is an indispensable element as a basic element for improving the strength of the base metal in steel, and as an effective lower limit, addition of 0.01% or more is necessary, but an excess exceeding 0.20% Addition causes a decrease in the weldability and toughness of the steel material, so the upper limit was made 0.20%.
Si: Si is an element necessary as a deoxidizing element in steelmaking, and 0.02% or more is necessary to be added to the steel. However, if it exceeds 0.5%, the HAZ toughness is lowered, so that is the upper limit. .
Mn: Mn is an element necessary for ensuring the strength and toughness of the base material. However, if it exceeds 2.0%, the HAZ toughness is remarkably impaired, but if it is less than 0.8 %, the strength of the base material is secured. Therefore, the range is set to 0.8 to 2.0%.
[0014]
P: P is an element that affects the toughness of steel, and if it exceeds 0.03%, not only the base material of steel but also the toughness of HAZ is significantly inhibited, so the upper limit of its content is 0.03%. did.
S: When S is added in excess of 0.030%, coarse sulfides are formed and toughness is inhibited. When the content is less than 0.0025 %, intragranular ferrite is formed. Since the amount of sulfides such as MnS that is effective for reducing significantly decreases, 0.0025 to 0.030% is made the range.
[0015]
V: V is a main element of the present invention, added as a nitride [VN] forming element, and acts as a nucleus of intragranular ferrite. At this time, the stoichiometric excess of N is added, so that the growth of grain boundary ferrite is remarkably suppressed by precipitation of carbide or carbonitride and securing of solid solution V. For these effects, addition of 0.07 % or less is not sufficient, and addition exceeding 0.50% causes a decrease in toughness. Therefore, the range is made 0.07 to 0.50% or less. .
[0016]
Al: Al is usually added as a deoxidizer, but in the present invention, if added over 0.05%, the effect of adding Mg and Ca is inhibited, so this is the upper limit. Further, 0.0005% is necessary to stably produce Mg and Ca oxides, and this is set as the lower limit.
Ti: Ti is an element that is effective as a deoxidizer and further as a nitride-forming element, and is effective in reducing the grain size. However, the addition of a large amount causes a significant decrease in toughness due to the formation of carbides. The upper limit needs to be 0.050%, but in order to obtain a predetermined effect, 0.003% or more must be added, and the range is made 0.003 to 0.050%.
[0017]
Mg: Mg is the main alloying element of the present invention, and is mainly added as a deoxidizer or sulfide-forming element, and remarkably suppresses the growth of intergranular ferrite by simultaneous addition with Ca. The amount is 0.0010 % in total with the Ca amount as the lower limit of the growth inhibition effect of the grain boundary ferrite, and the upper limit is 0.02 in total with the Ca amount because the toughness is reduced due to coarse oxide formation. 01%. When the amount of Mg alone exceeds 0.0025 %, coarse oxides or sulfides are likely to be formed, resulting in a decrease in the base material and HAZ toughness. On the other hand, if the addition is less than 0.0009 %, the formation of oxides necessary as pinning particles cannot be expected sufficiently, so the addition range is desirably limited to 0.0009 to 0.0025 %.
Ca: Ca suppresses the generation of stretched MnS by generating sulfides, and improves the properties in the thickness direction of the steel material, particularly the lamellar resistance. Furthermore, Ca is an important element of the present invention because it has the effect of suppressing the growth of grain boundary ferrite by the simultaneous addition with Mg as described above. For the same reason as the range of Mg, the range of Ca is desirably limited to a range of 0.0001% to 0.0085%, and further considering the relationship with the amount of Mg, the total is 0.0010 to 0%. Must be limited to 0.01%.
[0018]
In the present invention, as an element for improving strength and toughness, one or more elements among Cu, Ni, Cr, Mo, Nb, Zr, and B may be added as necessary. it can.
[0019]
Cu: Cu is an element effective in increasing the strength without reducing toughness, but if it is less than 0.05%, it is not effective, and if it exceeds 1.5%, it tends to cause cracking when heating the steel slab or welding. To do. Therefore, the content is made 0.05 to 1.5% or less.
Ni: Ni is an element effective for improving toughness and strength, and in order to obtain the effect, addition of 0.05% or more is necessary. However, addition of 0.53 % or more reduces weldability. Therefore, the upper limit is made 0.53 %.
Cr: Cr is effective to add 0.02% or more to improve the strength of steel by precipitation strengthening, but if added in a large amount, the hardenability is increased, the bainite structure is generated, and the toughness is reduced. . Therefore, the upper limit is made 1.5%.
[0020]
Mo: Mo is an element that improves hardenability and at the same time forms carbonitride to improve strength. To obtain the effect, addition of 0.02% or more is necessary. The addition of a large amount exceeding 50% brings about a remarkable decrease in toughness as well as an unnecessarily strengthening, so the range is made 0.02 to 0.50% or less.
Nb: Nb is an element that forms carbides and nitrides and is effective in improving the strength. However, the addition of 0.0001% or less has no effect, and the addition exceeding 0.20% reduces toughness. Therefore, the range is made 0.0001 to 0.20% or less.
Zr : Zr is also an element which forms carbides and nitrides and is effective in improving the strength like Nb. However, the addition of 0.0001% or less has no effect, and the addition of more than 0.050% In order to cause a decrease in toughness, the range is made 0.0001 to 0.050% or less.
[0021]
B: In general, B is an element that increases the hardenability when dissolved, but lowers the dissolved N as BN and improves the toughness of the heat affected zone. Therefore, the effect can be utilized by addition of 0.0003% or more, but excessive addition causes a decrease in toughness, so the upper limit is made 0.0050%.
[0022]
The steel containing the above components is subjected to reheating, rolling, and cooling through continuous casting after melting in the steelmaking process. In this case, the following points were limited.
In both hot rolling and controlled rolling, it is necessary to heat the steel ingot to a temperature of Ac 3 point or higher in order to austenite the steel ingot. However, if heating exceeds 1350 ° C., the heat source cost increases, so the heating temperature is set to 1350 ° C. or lower.
Next, in both hot rolling and controlled rolling, it is necessary to reduce the austenite grain size by rolling in the recrystallization temperature range. In addition, when using controlled rolling to increase strength and improve toughness, it is effective to introduce a deformation band into the austenite grains by rolling in the non-recrystallization temperature range and introduce ferrite transformation nuclei. is there. If the cumulative reduction rate in the non-recrystallized region is less than 40%, the deformation band is not sufficiently formed. Therefore, the lower limit value of the cumulative reduction rate in the non-recrystallized region is set to 40%. However, if the cumulative rolling reduction exceeds 90%, the absorbed energy in the base metal Charpy test is significantly reduced, so the upper limit was made 90%.
[0023]
Accelerated cooling is required to increase the strength further than natural cooling. However, if the cooling rate is less than 1 ° C./sec, sufficient strength cannot be obtained. On the contrary, when the cooling rate exceeds 60 ° C./sec, the toughness of the base material decreases because a bainite main structure is formed. Therefore, the cooling rate was limited to 1-60 ° C./sec. In the present invention, it is necessary to continue accelerated cooling until the transformation is completed in order to obtain the strength of the base material. For this reason, the upper limit of the cooling stop temperature was set to 600 ° C. Since the transformation does not end at a stop temperature exceeding 600 ° C., sufficient strength cannot be obtained. Usually, accelerated cooling uses water as a cooling medium. Therefore, since the lower limit of the actual cooling stop temperature is 0 ° C., the lower limit is set to 0 ° C.
[0024]
Since the tempering heat treatment after accelerated cooling is intended to improve the toughness of the base metal structure by recovery, the heating temperature must be not more than the Ac 1 point which is a temperature range in which reverse transformation does not occur. Recovery reduces the lattice defect density by the disappearance and coalescence of dislocations. In order to realize this, heating to 300 ° C. or higher is necessary. For this reason, the minimum of heating temperature was 300 degreeC. Since the upper limit is below the transformation point, AC 1 was set as the upper limit.
[0025]
【Example】
Next, examples of the present invention will be described.
A steel ingot having the chemical composition shown in Table 1 was made into a thick steel plate having a thickness of 12 mm to 100 mm according to the manufacturing conditions shown in Table 2. Then, the welding heat input was 100 kJ / mm, and the cooling rate at 800 ° C. to 500 ° C. was 0.00. A super heat input and super slow cooling welding of 35 ° C./s was applied to measure the particle sizes of old γ grains and intergranular ferrite, and the weld HAZ toughness was evaluated by Charpy absorbed energy at 0 ° C. In addition, about the base material toughness, although it manufactured at the temperature of 5 levels in the range of 1100 degreeC-1350 degreeC, all were favorable base material toughness. The ductile / brittle transition temperature (vTrs) of the invention steel was −40 ° C. or less, and showed a high Charpy absorbed energy value (100 J or more) in the range of the test temperature from −40 to −80 ° C. Next, the HAZ toughness of the weld will be described.
[0026]
First, steels 1 to 6 show examples of the present invention. As shown in Table 2, the steel sheet of the present invention satisfies the requirements of chemical composition and production conditions, and the HAZ heating γ grain size is 200 μm or less, and in addition, the old γ grain during cooling. It can be seen that the grain size of the grain boundary ferrite generated along the boundaries is 50 μm or less, and has high HAZ toughness.
[0027]
On the other hand, Steels 7 to 19 are comparative examples deviating from the method of the present invention. That is, Steels 7 to 11 are examples in which any one of the basic components or selected elements is added beyond the requirements of the invention, and securing the V amount, which is an important issue of the present invention, and “Mg and Although the requirement regarding the “total amount of Ca” is satisfied, excessive deterioration of the super heat input HAZ toughness is large due to the excessive addition of elements that cause toughness deterioration. In Steels 12 to 15 , both V and Al correspond to the case where they deviate from the lower limit value or the upper limit value. Looking at the characteristics in order, first, the steel 12 has a low toughness due to the large grain boundary ferrite. It can be seen that the toughness value of the steel 13 is deteriorated because the amount of V is remarkably high. The steel 14 and the steel 15 have shown that the influence of Al amount is large, and also in these cases, toughness is low. Next, steel 16 to steel 18 are examples in which the total amount of Mg + Ca is out of range. Steel 16 is a case where the grain boundary ferrite is coarsened due to the lack of these elements, while steels 17 and 18 have a grain boundary ferrite of not less than 5 μm despite being excessively added. The increase in the number of coarse oxides greatly reduces the toughness value.
Steel 19 is a case where none of the three elements V, Mg, and Ca is added, and grain boundary ferrite is remarkably coarsened compared to the other, and the HAZ toughness is the worst.
[0028]
[Table 1]
[Table 2]
【The invention's effect】
By limiting to the chemical components and the production method of the present invention and adding Mg and Ca simultaneously, the heated γ particle size of the base material can be refined, and further, regardless of welding heat input by combination with V addition Grain boundary ferrite that adversely affects toughness as well as heated HAZ grain size of HAZ can be refined at the same time, and these refinement effects make it an epoch-making high in both base metal toughness and welded HAZ toughness It is possible to produce high-strength welded structural steel. As a result, the safety against brittle fracture of welded structures such as buildings, bridges, shipbuilding, offshore structures, line pipes and construction machinery is greatly improved, and the industrial effect is remarkably great.
Claims (7)
Ｃ ：０．０１〜０．２０％
Ｓｉ：０．０２〜０．５０％
Ｍｎ：０．８５〜２．０％
Ｐ ：≦０．０３％
Ｓ ：０．００２５〜０．０３０％
Ｖ ：０．０７〜０．５０％
Ａｌ：０．０００５〜０．０５０％
Ｔｉ：０．００３〜０．０５０％
を含み、残部が鉄および不可避的不純物からなり、さらに、ＭｇとＣａを同時に含有し、それぞれが、Ｍｇは０．０００９〜０．００２５％の範囲、Ｃａは０．０００１〜０．００８５％の範囲で、且つ、その合計が０．００１０〜０．０１０％の範囲であることを特徴とする超大入熱溶接部ＨＡＺ靭性に優れた高強度溶接構造用鋼。In mass% C: 0.01 to 0.20%
Si: 0.02 to 0.50%
Mn: 0.85 to 2.0%
P: ≦ 0.03%
S: 0.0025 to 0.030%
V: 0.07 to 0.50%
Al: 0.0005 to 0.050%
Ti: 0.003 to 0.050%
And the balance is made of iron and inevitable impurities, and further contains Mg and Ca at the same time, each of Mg is in the range of 0.0009-0.0025%, Ca is 0.0001-0.0085% A high strength welded structural steel excellent in super high heat input weld HAZ toughness, characterized by being in a range and a total of 0.0010 to 0.010%.
Ｃｕ：０．０５〜１．５０％
Ｎｉ：０．０５〜０．５３％
Ｃｒ：０．０２〜１．５０％
Ｍｏ：０．０２〜１．５０％
Ｎｂ：０．０００１〜０．２０％
Ｚｒ：０．０００１〜０．０５０％
Ｂ ：０．０００３〜０．００５０％
のうち１種または２種以上を含有することを特徴とする請求項１記載の超大入熱溶接部ＨＡＺ靭性に優れた高強度溶接構造用鋼。In mass%,
Cu: 0.05 to 1.50%
Ni: 0.05-0.53%
Cr: 0.02-1.50%
Mo: 0.02 to 1.50%
Nb: 0.0001 to 0.20%
Zr: 0.0001 to 0.050%
B: 0.0003 to 0.0050%
The high-strength welded structural steel having excellent high heat input welded HAZ toughness according to claim 1, comprising one or more of them.
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